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Counting days Carbon-dating of an ancient beam from a Guatemalan temple may help end a century-long debate about the Mayan calendar, say anthropologists.

Experts have long wrangled over how the Mayan calendar correlates to the European calendar.

Ancient texts and carvings from the Mayan culture describe rulers and great events and attribute the dates according to a complex system denoted by dots and bars, known as the Long Count.

"The Long Count calendar fell into disuse before European contact in the Maya area," says the study's lead author Douglas J Kennett from Pennsylvania State University.

"Methods of tying the Long Count to the modern European calendar used known historical and astronomical events, but when looking at how climate affects the rise and fall of the Maya, I began to question how accurately the two calendars correlated using those methods," says Kennett.

The study, published in Scientific Reports , backs up previous carbon-dating studies, and supports the hypothesis that climate change played an important role in the development and demise of this complex civilisation, say the researchers.

But the date of this starting point is unknown. Spanish colonisers did their utmost to wipe out traces of the Mayan civilisation, destroying evidence that could have provided a clue.

An example of the confusion this has caused is the date of a decisive battle that shaped the course of Mayan civilisation.

It occurred at nine Bak'tuns, 13 K'atuns, three Tuns, seven Winals and 18 K'ins -- or 1,390,838 days from the start of the count. Attempts to transcribe this into the European calendar have given estimates that vary by hundreds of years.

To pin the date of the battle down, Kennett and colleagues carbon-dated a tiny sample from a carved wooden lintel found at a temple in the city of Tikal, the centre of the now-vanished Mayan civilisation.

The team concluded the tree was cut down and carved around AD 658-696.

Battle date

The estimate closely matches that of a decades-old benchmark for Mayan dating, the so-called Goodman-Martinez-Thompson method.

According to the Goodman-Martinez-Thompson estimate, the big victory occurred around AD 695-712.

That figure was bolstered by early use of carbon-14 dating on two other wooden beams from Tikal in the 1950s.

The small discrepancy between the two dates may find an explanation in the wood itself, Kennett's team believes.

They say the huge lintel was taken from a tree called the sapotilla (Manilkara zapota), which has a very hard wood and would have taken years to strip and carve using stone-age implements.

Armed with two good fixes on the date of this decisive battle, historians should be able to build a more accurate chronology of the rise and fall of the Mayan civilisation, say the researchers.

"We can now argue with greater certainty that Jasaw Chan K'awill's accession to Tikal's throne occurred in AD 682 and this decisive victory over Calakmul occurred in AD 695," they write.

"These events and those recorded at cities throughout the Maya lowlands can now be harmonised with greater assurance to other environmental, climatic and archaeological data from this and adjacent regions."

The ancient Mayans reached the peak of power in Central America between AD 250 and 900.

The civilisation's sudden collapse has also been hotly debated. Mooted causes include disease, foreign invasion and deforestation or climate change that led to a prolonged, devastating drought.